[Show abstract][Hide abstract] ABSTRACT: Chestnut-like structured carbon comprising platelet carbon nanofibers(PCNFs) grown on selective catalytic gasified activated carbon have shown promising results as application for electrodes. The Ni-Fe catalyst is generally prepared on the activated carbon by immersion process followed by a reduction of temperature at 350-450 degrees C. The growth of PCNFs then continue on for a predetermined time through the thermal decomposition of ethylene at 600 degrees C. The resulting structure, which comprises an intimately connected activated carbon and PCNFs, is shown to offer performance advantages with its specific surface properties and electrochemical characterizations. The mesoporous volume is 0.7564 cm(3)/g higher than that with purely activated carbon (0.1220 cm(3)/g). The specific surface area is 2401 m(2)/g higher than those with purely activated carbon (1800 m(2)/g). The specific capacitance is 136.86 F/cm(3) higher than those with P-60.
[Show abstract][Hide abstract] ABSTRACT: The effect of membrane thickness on the performance degradation of DMFCs was investigated during freeze-thaw cycling across temperatures ranging from -32 degrees C to 60 degrees C. Three cells with Nafion membranes of varying thickness were prepared: Nation 112, Nafion 115 and Nation 117. Performance degradation was evaluated by comparing the changes to electrode polarization, electrochemical impedance and cyclic voltammetry over a range of freeze-thaw cycles. It was determined that freeze-thaw cycling affected the performance of the three membrane electrode assemblies (MEA). A cell with a Nation 112 membrane showed a more significant increase in cathode overpotential than cells with either a Nation 115 or Nafion 117 membrane. The charge transfer resistance of the cell with a thin membrane was more affected by freeze-thaw cycles than the cells with thicker membranes. All three cells showed a significant decrease in ECSA after freeze-thaw cycles. Freeze-thaw cycles damaged the triple phase boundary region and decreased the ECSA of cells with thinner membranes, which cause rapid performance degradation. The use of thick membranes in MEAs was determined to be the effective method for reducing performance degradation during freeze-thaw cycling. Copyright
International Journal of Hydrogen Energy 09/2014; 39(28):15760-15765. DOI:10.1016/j.ijhydene.2014.06.031 · 2.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tungsten carbides (WC) nanoparticles on platelet type-carbon nanofibers (p-CNFs) catalysts have been synthesized for alkaline direct ethanol fuel cells (ADEFC). Physical properties of WC/CNFs samples with various WC contents are analyzed by transmission electron microscope (TEM), thermal gravimetric analysis (TGA) and nitrogen isotherm (BET). The WC/CNFs catalysts showed an improved kinetics for the ethanol oxidation than p-CNFs did. It indicates that the significant increase in the catalytic activity for ethanol oxidation on WC/CNFs than p-CNFs did due to the synergistic structural effect between WC nanoparticles and the p-CNFs supports. WC/CNFs also showed good performances in ADEFC single cells. The maximum current density of P4W3 and P4W4 was 9.0 and 4.4 mA cm(-2), respectively. These catalysts can be used as the ethanol oxidation in direct ethanol fuel cells in alkaline media. Copyright
International Journal of Hydrogen Energy 09/2014; 39(28):15907-15912. DOI:10.1016/j.ijhydene.2014.02.010 · 2.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Proton exchange membranes were prepared by γ-ray radiation-induced grafting onto ETFE films, crosslinking with styrene/divinylbenzene (DVB) mixtures and a subsequent sulfonation. Effects of the DVB content on the membrane properties such as ion exchange capacity (IEC), proton conductivity, and methanol permeability were investigated. Membrane-electrode assemblies (MEA) containing the prepared membranes were fabricated for direct methanol fuel cells (DMFC), and the MEA performance was examined in operation with various concentrations of methanol. Durability of the MEAs was investigated using a 3 M methanol solution for 1000 h, and the polarization curves and impedance spectra were measured. Physicochemical properties of catalysts and membrane before and after the long-term operation were analyzed by TEM, XRD, IEC, and proton conductivity. Performance and durability of the selected membranes were also confirmed in the operation of a 6-cell stack with 3 M methanol solution.
[Show abstract][Hide abstract] ABSTRACT: This paper reports the effect of adding reduced graphene oxide (RGO) as a conductive material to the composition of an electrode for capacitive deionization (CDI), a process to remove salt from water using ionic adsorption and desorption driven by external applied voltage. RGO can be synthesized in an inexpensive way by the reduction and exfoliation of GO, and removing the oxygen-containing groups and recovering a conjugated structure. GO powder can be obtained from the modification of Hummers method and reduced into RGO using a thermal method. The physical and electrochemical characteristics of RGO material were evaluated and its desalination performance was tested with a CDI unit cell with a potentiostat and conductivity meter, by varying the applied voltage and feed rate of the salt solution. The performance of RGO was compared to graphite as a conductive material in a CDI electrode. The result showed RGO can increase the capacitance, reduce the equivalent series resistance, and improve the electrosorption capacity of CDI electrode.
[Show abstract][Hide abstract] ABSTRACT: Poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether)-graft-poly(styrene sulfonic acid) (PFA-g-PSSA) membranes were prepared by γ-ray radiation-induced grafting of styrene onto 50μm PFA films and a subsequent sulfonation. The properties of the PFA-g-PSSA membranes such as ion exchange capacity, water uptake, proton conductivity, and methanol permeability were evaluated and compared with commercial Nafion 115 membrane. The characteristic properties and direct methanol fuel cell (DMFC) performances with PFA-g-PSSA membrane were better than that with the Nafion 115 membrane. The power density of the single cell with PFA-PSSA30, PFA-PSSA50, PFA-PSSA80 membranes having different degree of grafting (DOG), and Nafion 115 membranes was 113, 123, 118, and 111mWcm-2 at 0.4V, respectively. To study the long-term durability of the prepared membrane electrode assembly (MEA) with a PFA-PSSA50 membrane, the single cell was tested for 2066h in a galvanostatic mode under a constant load 150mAcm-2 and at a temperature of 60°C. The performance loss of the MEA after 2066h operation was about 26.8%. Before and after the long-term performance test, the degradation of MEA with PFA-g-PSSA membrane was investigated by polarization curve technique, electrochemical impedance spectroscopy, TEM, X-ray diffraction, and ion exchange capacity.
[Show abstract][Hide abstract] ABSTRACT: Effects of a CrN/Cr coating layer on the durability of metal bipolar plates (stainless steel (STS) 430) are investigated in direct methanol fuel cells (DMFCs) with a fuel recirculation system, since under fuel recirculation the metal bipolar plates can be faced with a tougher corrosion environment. Before the fuel recirculation, the performance losses of the cells consisting of metal bipolar plates are ascribed to cathode degradation, due to a high corrosion level of the cathode side. However, after fuel recirculation, corrosion of the anode metal bipolar plate by pH decrease and overpotential increase brings about severe anode degradation. It is found that damage by corrosion of the cathode metal bipolar plate is limited to degradation of the cathode catalyst, whereas corrosion of the anode metal bipolar plate deteriorates not only the catalysts but also the electrolyte membrane. These durability tests show that the CrN/Cr coatings deposited on the STS 430 improve the corrosion resistance of the metal substrate and lead to low performance degradation. Copyright
International Journal of Hydrogen Energy 08/2013; 38(25):10567-10576. DOI:10.1016/j.ijhydene.2013.06.011 · 2.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To investigate the effects of the microstructure and powder compositions for the micro-porous layer (MPL) of an anode on the cell performance of a direct methanol fuel cell (DMFC) using a highly concentrated methanol solution up to 7 M, various powders and their compositions were applied as a filler of the MPL in the membrane electrode assembly (MEA). Several nano- and microstructured carbons such as commercial carbon black (CB), spherical activated carbon (AC), multi-walled carbon nanotube (MWCNT), and platelet carbon nanofiber (PCNF) were selected with different morphology and surface properties, and a meso-porous silica (one of SBA series) was also included for its porous and hydrophilic properties. The coating morphology and physical properties such as porosity and gas permeability were measured, and electrochemical properties of MEA with the MPL were examined by using current–voltage polarization, electrochemical impedance spectroscopy, and voltammetric analyses. A mixture of different carbons was found to be effective for lowering methanol crossover with sustaining electrical conductivity and gas permeability. A MEA with modified-anode MPLs made of CB (50 vol%) and PCNF (50 vol%) powders showed a maximum power density of 67.7 mW cm−2 under operation with a 7 M concentration of methanol.
International Journal of Hydrogen Energy 06/2013; 38(17):7159–7168. DOI:10.1016/j.ijhydene.2013.04.003 · 2.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Freeze-thaw cycles were used to investigate performance degradation in direct methanol fuel cells (DMFC). The freeze-thaw cycles were carried out across the temperature range of −32 °C–60 °C. The details of the performance degradation were analyzed by comparing the change of polarization of each electrode and the electrochemical impedance spectrum according to the number of freeze-thaw cycles. It was found that freeze-thaw cycles caused the increase in the cathode overpotential to affect performance degradation and the increase in the charge transfer resistance which means distinct damages in the triple phase boundary of the catalyst layer. Different purging scenarios before freezing were adopted, namely the cathode purge and the anode–cathode purge, to reduce any performance degradation caused by the freeze-thaw cycles. The cells purged by nitrogen gas were found to have less performance loss than the cells that were not purged during the freeze-thaw cycles. The changes in the cell resistance and the cathode electrochemical surface areas were also smaller when the cells were purged compared with those cells that were not purged. The introduction of air purging had similar positive influences with nitrogen purging on the performance of the DMFCs and their impedance. It was also determined that air was better at purging only the cathode than purging both electrodes.
International Journal of Hydrogen Energy 11/2012; 37(22):17268–17274. DOI:10.1016/j.ijhydene.2012.08.081 · 2.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sulfonated SiO2 nanoparticles with uniform core–shell structure are synthesized and used as functional fillers for preparing composite proton conducting membranes for direct methanol fuel cells (DMFCs). Poly(4-styrenesulfonic acid) in the shell of SiO2 nanoparticle contributes well-dispersion of the SiO2 nanoparticle in the Nafion membrane. The addition of core–shell SiO2 nanoparticles into Nafion matrix is very effective in improving membrane performance, including ion exchange capacity, proton conductivity, mechanical strength and methanol permeability. As a result, the DMFC assembled with a composite membrane exhibits superior performance as compared to a Nafion-based cell.
Journal of Membrane Science 10/2012; s 415–416:696–701. DOI:10.1016/j.memsci.2012.05.057 · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study experimentally and numerically investigates the effects of the four different types of serpentine flow-field geometry on the performance of a direct methanol fuel cells (DMFCs). To elucidate the effect of different channel/rib aspect ratios, the through/in-plane transport of methanol and air, the current density distribution, the anode and cathode polarization, the impedance and the cathode pressure drop are numerically and/or experimentally observed. The sub-rib convection significantly affects the cell performance with the serpentine flow fields. Thus, a narrow rib width and a suitable channel/rib aspect ratio are needed to increase the cell performance of serpentine DMFCs. This flow channel, for example, an F2-type flow channel (channel width: 1.0 mm, rib width: 0.5 mm), leads to formation of a narrower rib chain as well as a higher current density distribution in the rib, and thereby it brings about improvement of the two-phase flow in DMFCs, which is supported through the high performance difference in the high current density regions and the long-term stability test of 700 h.
Journal of Power Sources 05/2012; 205:32–47. DOI:10.1016/j.jpowsour.2011.12.055 · 6.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The chemical stability and durability of PtRu catalysts supported on carbon nanofibers (CNFs) for the anode electrode of a direct methanol fuel cell (DMFC) are investigated by Pt and Ru dissolution tests in sulfuric acid and long-term performance tests of a single cell discharging at a constant current density of 150 mA cm−2 for approximately 2000 h. A CNF with a herringbone-type structure, which is characterized by the alignment of graphene symmetrically angled to the fiber axis, was selected as the catalyst support because it has an edge-rich surface and a high surface area. In the metal dissolution test, the PtRu/CNF catalysts showed 1.5–2 times lower Ru leaching than a tested commercial catalyst (supported on activated carbon). The results of long-term performance tests also prove the higher durability of the anode catalyst compared with the commercial catalyst, when the anode polarization is compared before and after operation for 2000 h. Some analytical measurements, including X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy were conducted to study the degradation of the catalyst activity.
Fuel and Energy Abstracts 03/2012; 37(5). DOI:10.1016/j.ijhydene.2011.04.119
[Show abstract][Hide abstract] ABSTRACT: The outer micro-porous layer (MPL) between the gas diffusion layer and channel of the bipolar plate was studied for both sides of the electrodes in DMFC, with particular attention to the effects of the hydrophobicity of the MPL on mass transport as well as cell performance. Water-transport behavior from the electrodes to the channel was observed through the transparent window of the single cell with membrane-electrode assemblies (MEAs) including three combinations of outer MPLs. The crossover amount of methanol as well as water through the membrane was measured, and the mass balance, based on the measured flux, was established to understand the mass transport in MEAs. The design of outer MPLs is discussed for the best cell performance.
Fuel and Energy Abstracts 03/2012; DOI:10.1016/j.ijhydene.2011.04.159
[Show abstract][Hide abstract] ABSTRACT: Various silica particles were adopted as catalyst supports, and silica-supported PtRu catalysts were evaluated as catalysts for the anode of direct methanol fuel cells at methanol concentrations of 1–10 M through single cell tests. Compared to a carbon black supported Pt–Ru catalysts, the silica-supported Pt–Ru catalysts exhibited higher performance in MEA, especially with high concentration over 3 M, and the maximum power density reached to 90 mW cm−2 and 60 mW cm−2 with 5 M and 10 M, respectively, which were 1.5 and 3 times higher than the reference carbon black supported catalysts. It was found that the silica particles as a catalyst support have a significant effect on reduction of methanol crossover and control of fuel feeding. Such a high performance in the operation with high concentrations was confirmed in the long-term durability test.
Fuel and Energy Abstracts 03/2012; 37(5). DOI:10.1016/j.ijhydene.2011.05.068
[Show abstract][Hide abstract] ABSTRACT: According to the conventional MEA test, methanol and water crossover are the main factors to determine performance of a passive DMFC. Thus, to ensure the high cell performance of a passive DMFC using high concentration methanol of 50–95 vol%, the MEA in this study introduces the barrier layer to limit the crossover of high concentration methanol, a hydrophobic layer to reduce water crossover, and a hydrophilic layer to enhance the water recovery from the cathode to the anode. The functional layers of the MEA have the effect of improving the performance of the passive DMFC by decreasing the methanol and water crossover. In spite of the operation with 95 vol% methanol, the MEA with multi-layer electrodes for high concentration methanol DMFCs shows a maximum power density of 35.1 mW cm−2 and maintains a high power density of 30 mW cm−2 (0.405 V) under constant current operation.
Fuel and Energy Abstracts 03/2012; 37(5). DOI:10.1016/j.ijhydene.2011.04.112
[Show abstract][Hide abstract] ABSTRACT: Highly dispersed Pd nanoparticles were prepared by borohydride reduction of Pd(acac)(2) in 1,2-propanediol at an elevated temperature. They were uniformly dispersed on carbon black without significant aggregation. X-ray diffraction showed that carbons from the Pd precursor dissolved in Pd, increasing its lattice parameter. A modified reduction process was tested to remove the carbon impurities. Carbon removal greatly enhanced catalytic activity toward the oxygen reduction reaction. It also generated an inconsistency between the electronic modifications obtained from X-ray photoelectron spectroscopy and the electrochemical method. CO displacement measurements showed that the formation of Pd-C bonds decreased the work function of the surface Pd atoms.
[Show abstract][Hide abstract] ABSTRACT: In this work, we studied the characteristic variations of catalyst supports caused by mechanical milling and their electrochemical application in fuel cells. Two different catalyst supports, carbon black (XC-72R) and K20 (mesoporous carbon), were crushed and dispersed by mechanical milling using a bead mill. The bead mill operated with 0.3μm zirconia beads at the rate of 3500rpm for 30min. The secondary particle size of the crushed catalyst supports ranged from around 0.1μm to 10μm. The secondary particle size of the catalyst supports after crushing represents a decrease of approximately 10% compared with that of raw catalyst supports. To confirm the role of the catalyst supports in the direct methanol fuel cell (DMFC), Pt and Ru were loaded onto these catalyst supports using an impregnation method. In the single cell test, Pt–Ru/XC-Bead and PtRu/K20-Bead showed power densities of 135mW/cm2 and 144mW/cm2 under air at 60°C, respectively. The performance values of these catalysts, which were fabricated using reformed catalyst supports, were 10% to 20% higher than those of raw catalyst supports. As a result, the catalyst supports crushed by the bead mill helped to improve the electrochemical performance of the direct methanol fuel cell.
[Show abstract][Hide abstract] ABSTRACT: Porous carbon nanofibers(CNF) were synthesized via NaOH activation at 700~, and the porous CNF-supported PtRu catalysts were evaluated for the anode in direct methanol fuel cells. The change of surface characteristics by NaOH activation was examined by analyses of the specific surface area and pore size distribution. The morphological and structural modification was investigated under scanning electron microscopy. The activity of catalysts supported on porous CNFs was examined by cyclic voltammograms and single cell tests. The pore formation on CNF by the NaOH activation was discussed, concerning the catalyst activity, when they were applied as catalyst supports.
Korean Chemical Engineering Research 12/2011; 49(6). DOI:10.9713/kcer.2011.49.6.769